The present invention relates to a bendable sheet material having a sintered core provided at both sides with duefile cladding sheets bonded to the core, and exemplified by a neutron absorbing panel which is an improvement over prior U.S. Pat. No. 4,027,377.
The prior patent discloses a panel having a core composed of a sintered mixture of particles of a neutron absorbing material such as boron carbide (B.sub.4 C) and aluminum having cladding sheets of metal, such as aluminum at opposite sides. The cladding sheets are of uniform thickness so that the core is centered with respect to the median plane of the panel.
Panels of this construction may be assembled into enclosures for neutron emitting material, as disclosed for example in prior U.S. Pat. No. 4,006,362, but it is desirable in some cases to bend the panels to form rounded corners connecting flat panel sections, or bending a flat strip into a tube.
Since the core material lacks the ductility of the cladding sheaths, bending a flat panel or strip into curved or arcuate form has resulted in disturbance of the integrity of the neutron absorbing core.
In protective sheet material of the type disclosed here, it is essential that the neutron absorbing core is uniformly effective to minimize or prevent escape of harmful or lethal neutrons. Accordingly, any disturbance of the core in the bent zone which results in areas of reduced neutron absorbing effectiveness is a serious defect.
It has been found that such disturbance of the neutron absorbing core by bending is reduced by providing a cladding sheet at the side of the panel which is remote from the axis of the bend which has a thickness greater than that of the cladding sheet at the other side of the panel. The result is that the core is displaced from the central or median plane of the panel. The central plane of the panel may be considered as the neutral plane in the bending step, with the result that the material of the panel at the side of the neutral plane adjacent the bending axis is under compression, while the material at the other side of the neutral plane is under tension and is stretched or elongated as a result at the bending step. The cladding sheet, such as aluminum a is ductile, and is progressively stretched or elongated and correspondingly thinned at the side of the neutral plane remote from the axis of the bend. The core material lacking the ductility of the cladding material, tends to be cracked or fissured at the side of the neutral plane remote from the axis of bending.
On the other hand, the core material at the side of the neutral plane adjacent the axis of bending is subjected to compressive forces which it can accommodate without loss of integrity, so far as its neutron absorbing capability is concerned.